Independent rear suspension systems have gotten a bit of a bad rap in the aftermarket industry mainly due to the outdated and poorly engineered Jaguar-style IRS design. The Jaguar-style suspension does not utilize an upper control arm, and retains the spindle and wheel location by relying on a half-shaft–style axle. Should a half-shaft or U-joint break, the wheel of the car is free to move inward and outward uncontrollably, causing a very dangerous situation. Without using an upper control arm to retain the spindle and hub assembly you are very limited to the horsepower you can apply. Imagine the massive twisting force a rear tire exerts when it is under had acceleration. If there is no upper link to counteract this force, you are relying on the lower suspension pivot for stability, which is now seeing this twisting force amplified by the leverage effects of the suspension. On top of the lack of strength in these systems, there are no performance gains to be had with them. The camber gains are very poor and often produce positive camber, the bushing and suspension hardware have massive amounts of deflection, and there is no antisquat available to assist in traction. There are many companies that build very high-end, Jaguar-based suspensions that are well suited to lightweight street rods and are very aesthetically pleasing for show cars and cruisers. For the Pro Touring enthusiast, however, it is mandatory that an IRS system use a full upper A-arm assembly. The strength and performance benefits that come with this design simply cannot be matched with a Jaguar-based unit.
Many traditionalists fear an IRS because they feel they're too weak to handle high-horsepower levels, and this is mainly due to the poor strength and performance of Jaguar-based IRS systems. We have gone to great lengths to make sure that the RS Fast Track IRS can withstand massive amounts of horsepower. For instance, we have utilized the best materials in our CV axles that start with an unbreakable 300M axleshaft using specially machined chromoly, high-performance CV joints. These axles have been tested in drag race applications with more than 1,400 hp. We offer upgraded shafts and higher spline outer bearing assemblies to support horsepower numbers in excess of 2,500. Likewise, our control arms are made out of 0.125-inch wall DOM, 1.625-inch diameter tubing with Acetyl bushings that will not deflect under hard acceleration. Also, our IRS cradle has been CAD designed to withstand the forces of high-horsepower motors.
Our Fast Track IRS is available on any Roadster Shop chassis, as a stand-alone cradle, or as a custom-built back-half kit. It can be a very easy installation when it is integrated into a complete RS chassis. On some applications, rear floor modifications are required to accept the IRS system. While many RS chassis are designed to be direct bolt-ons to original bodies, most original bodies were not designed to accommodate an IRS. We simply could not let the factory floor limit the capabilities of the suspension from a strength, performance, and handling standpoint. In most cases, simple sheetmetal work is required for the installation. With the ragged condition of the average muscle car project and the rust and panel repair they require, this would not be outside the realm of anyone doing a restoration. We also offer our IRS as a stand-alone suspension cradle that can be installed by any shop or homebuilder. Fabrication skills are required, but the installation is simplified since the suspension is completely assembled in a stand-alone cradle. When weighing out the performance versus ease of installation compromise, we simply could not come to terms with sacrificing strength, performance, or ride quality simply so it could be a bolt-on part. As a result, it is not offered as a direct bolt-in application. The floors and subrails of muscle cars were never designed to accept an IRS, and the decision was made that like the front suspension, this could not be a limiting factor. So much would have to be changed, in fact, that the design compromises would have negated the benefits of building an IRS in the first place.
To maintain the quality and performance of our suspension products, we utilize a vast amount of modern equipment and technology. Every component is individually designed and engineered in 3-D modeling and tested for strength and fitment. The suspension geometry is extensively designed and mapped out in a complex suspension design and analysis program. All framerail and crossmember components are designed in either 2-D or 3-D software and then cut on a combination of high-definition CNC plasma machines and lasers. Components are bent using heavy tonnage CNC press brakes. To make sure our control arms maintain exact tolerances, we use CNC mandrel benders and have the tubes cut and notched by CNC pipe lasers. We have over 40 application-specific machined components that are CNC cut on four-axis milling machines. All of our components and assemblies are designed and tested in Solidworks 3-D software to make sure everything is up to par prior to building any prototypes. Additionally, heavy-duty fixtures are used to put all the components together once the sub-assemblies are complete. All of this technology allows us to produce high-quality, high-performance products over and over again.